The present invention relates to a process for the production of a composite material made unoxidizable at high temperature (up to 1800.degree. C. under a low air pressure), as well as to the material obtained by this process.
This material is more particularly intended for use as a high performance heat protection for space vehicles (shuttles or aircraft) having to withstand the heating caused by the friction of the air when they reenter the earth's atmosphere at high speed.
However, the invention is also applicable to other industrial fields requiring the use of structures able to withstand high mechanical stresses under temperatures above 1100.degree. C. in a corrosive medium.
The oxidation-protected materials to which the invention applies are composite materials, particularly of the carbon-carbon (C/C) type appropriately constituted by carbon fibers embedded in a carbon-based matrix.
One of the essential advantages of carbon-carbon materials is that they retain their integrity up to 3000.degree. C. or higher, under rapid heating. However, their major disadvantage is that they significantly oxidize as from 400.degree. C. in the presence of air.
Different processes have been envisaged to prevent this oxidation, such as the protection of the composite materials based on the use of a silicon carbide (SiC) coating formed on the outer surface of the carbon-containing parts. The use of this outer SiC coating is in particular described in FR-A-2 611 198 filed in the name of the present Applicant and documents EP-A-0 133 315, U.S. Pat. Nos. 3,095,316, 3,406,044, and 3,925,577.
The different deposition methods for a SiC layer on C/C composite materials always lead to the obtaining of a cracked layer as a result of the expansion coefficient variation between carbon and silicon carbide. To obviate this disadvantage, with the outer SiC coating is associated a silica or borosilicate glass coating for sealing the cracks of the SiC coating.
This sealing functions correctly up to approximately 1700.degree. C. under atmospheric pressure. However, under reduced pressure, the operating temperature of these materials is limited by the reaction of SiO.sub.2 on SiC corresponding to the equation SiC+SiO.sub.2 .fwdarw.2SiO+CO.
In order to ensure a sealing of the cracks of the SiC coating by a material able to withstand high temperatures under reduced pressures, the Applicant has envisaged depositing on the silicon carbide layer an outer oxide layer chosen from among ThO.sub.2, ZrO.sub.2, HfO.sub.2, La.sub.2 O.sub.3, Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 and an intermediate layer serving as a reaction barrier between the SiC and the oxide, said intermediate layer being chosen from in particular aluminum nitride and hafnium nitride. This arrangement is described in FR-A-2 635 773 filed on 31.8.1988 by the present Applicant.
The oxidation protection described in said document is completely satisfactory. However, for certain applications said protection is too complex. Moreover, it leads to a relatively heavy material. In addition, it would be of interest to have a more easily obtained, lighter material, in particular making it possible to eliminate the outer silicon carbide coating.